The adoption of an electromagnetic induction heating scheme into a fixing apparatus used for a copier, facsimile, printer or the like is actively being studied in recent years. As for the fixing apparatus based on an electromagnetic induction heating scheme, an AC current is applied to an excitation coil, and magnetic flux which repeats generation and annihilation is generated around this excitation coil. Then, an eddy current is generated when the generated magnetic flux passes through an electric conductor, and heat produced in the electric conductor by this eddy current is used for the fixing of an unfixed image.
More specifically, heat produced in the electric conductor is transferred to a nip formed by, for example, two rollers, and, when a recording material passes through the nip, toner on the recording material is fixed by the pressure of the rollers and the transferred heat. To transfer the heat produced in the electric conductor to the nip, for example, the roller which forms the nip may be formed with an electric conductor, or a thin film belt may be looped over the electric conductor and one of the rollers which form the nip.
By the way, the heat transferred to the nip is deprived by the recording material which passes through the nip and surrounding members, and the temperature of the roller or the belt which transfer the heat to the nip decreases. At this time, the width of the recording material which passes through the nip varies over a wide range, and heat is not always deprived from the whole widths of the roller and the belt uniformly.
That is, when a roller scheme whereby the roller forming the nip is formed with a electric conductor is taken as an example, the whole roller width of the heating roller formed with the electric conductor does not always contact the recording material at the nip, and, when a narrow recording material passes through the nip, heat is not deprived from a part which has no contact with the recording material. Therefore, the temperature outside the width of the recording material of the heating roller may increase excessively. When a wide recording material passes in such a condition that the temperature at that part exceeds the temperature suitable for fixing of the toner, a “hot offset” occurs, that is, the toner transferred to the recording material adheres to the heating roller again. This may also significantly shortens the life of the rubber member or the like which contacts the heating roller.
To cope with the problem of such an excessive temperature rise, it is possible to perform self temperature control using a temperature compensator alloy for which a Curie temperature is set as the electric conductor. The “Curie temperature” is a temperature which becomes a threshold for the presence/absence of magnetism of a temperature compensator alloy, and even a temperature compensator alloy which has ferromagnetism at a normal temperature loses the magnetism at a temperature exceeding the Curie temperature. Using the characteristic of such a temperature compensator alloy, it is possible to reduce an eddy current at the Curie temperature or above and suppress generation of heat by using a material whose Curie temperature is equal to a fixing temperature as the material of the conductive layer of the heat generating film as disclosed in Patent Document 1, for example.
Patent Document 1: Japanese Patent Application Laid-Open No. HEI 7-114276